Electrochemical Investigations of the Interconversions between Catalytic and Inhibited States of the [FeFe]-Hydrogenase from Desulfovibrio desulfuricans

Second and Outer Coordination Sphere Effects in Nitrogenase, Hydrogenase, Formate Dehydrogenase, and CO Dehydrogenase

Gases like H2, N2, CO2, and CO are increasingly recognized as critical feedstock in "green" energy conversion and as sources of nitrogen and carbon for the agricultural and chemical sectors. However, the industrial transformation of N2, CO2, and CO and the production of H2 require significant energy input, which renders processes like steam reforming and the Haber-Bosch reaction economically and environmentally unviable. Nature, on the other hand, performs similar tasks efficiently at ambient temperature and pressure, exploiting gas-processing metalloenzymes (GPMs) that bind low-valent metal cofactors based on iron, nickel, molybdenum, tungsten, and sulfur. Such systems are studied to understand the biocatalytic principles of gas conversion including N2 fixation by nitrogenase and H2 production by hydrogenase as well as CO2 and CO conversion by formate dehydrogenase, carbon monoxide dehydrogenase, and nitrogenase. In this review, we emphasize the importance of the cofactor/protein interface, discussing how second and outer coordination sphere effects determine, modulate, and optimize the catalytic activity of GPMs. These may comprise ionic interactions in the second coordination sphere that shape the electron density distribution across the cofactor, hydrogen bonding changes, and allosteric effects. In the outer coordination sphere, proton transfer and electron transfer are discussed, alongside the role of hydrophobic substrate channels and protein structural changes. Combining the information gained from structural biology, enzyme kinetics, and various spectroscopic techniques, we aim toward a comprehensive understanding of catalysis beyond the first coordination sphere.

Electrochemical Characterization of a Complex FeFe Hydrogenase, the Electron-Bifurcating Hnd From Desulfovibrio fructosovorans

Hnd, an FeFe hydrogenase from Desulfovibrio fructosovorans, is a tetrameric enzyme that can perform flavin-based electron bifurcation. It couples the oxidation of H₂ to both the exergonic reduction of NAD⁺ and the endergonic reduction of a ferredoxin. We previously showed that Hnd retains activity even when purified aerobically unlike other electron-bifurcating hydrogenases. In this study, we describe the purification of the enzyme under O₂-free atmosphere and its biochemical and electrochemical characterization. Despite its complexity due to its multimeric composition, Hnd can catalytically and directly exchange electrons with an electrode. We characterized the catalytic and inhibition properties of this electron-bifurcating hydrogenase using protein film electrochemistry of Hnd by purifying Hnd aerobically or anaerobically, then comparing the electrochemical properties of the enzyme purified under the two conditions via protein film electrochemistry. Hydrogenases are usually inactivated under oxidizing conditions in the absence of dioxygen and can then be reactivated, to some extent, under reducing conditions. We demonstrate that the kinetics of this high potential inactivation/reactivation for Hnd show original properties: it depends on the enzyme purification conditions and varies with time, suggesting the coexistence and the interconversion of two forms of the enzyme. We also show that Hnd catalytic properties (Km for H₂, diffusion and reaction at the active site of CO and O₂) are comparable to those of standard hydrogenases (those which cannot catalyze electron bifurcation). These results suggest that the presence of the additional subunits, needed for electron bifurcation, changes neither the catalytic behavior at the active site, nor the gas diffusion kinetics but induces unusual rates of high potential inactivation/reactivation.

Viologen-modified electrodes for protection of hydrogenases from high potential inactivation while performing H2 oxidation at low overpotential

In this work we present a viologen-modified electrode providing protection for hydrogenases against high potential inactivation.

In this work we present a viologen-modified electrode providing protection for hydrogenases against high potential inactivation. Hydrogenases, including O₂-tolerant classes, suffer from reversible inactivation upon applying high potentials, which limits their use in biofuel cells to certain conditions. Our previously reported protection strategy based on the integration of hydrogenase into redox matrices enabled the use of these biocatalysts in biofuel cells even under anode limiting conditions. However, mediated catalysis required application of an overpotential to drive the reaction, and this translates into a power loss in a biofuel cell. In the present work, the enzyme is adsorbed on top of a covalently-attached viologen layer which leads to mixed, direct and mediated, electron transfer processes; at low overpotentials, the direct electron transfer process generates a catalytic current, while the mediated electron transfer through the viologens at higher potentials generates a redox buffer that prevents oxidative inactivation of the enzyme. Consequently, the enzyme starts the catalysis at no overpotential with viologen self-activated protection at high potentials.

Methodologies for "Wiring" Redox Proteins/Enzymes to Electrode Surfaces

The immobilization of redox proteins or enzymes onto conductive surfaces has application in the analysis of biological processes, the fabrication of biosensors, and in the development of green technologies and biochemical synthetic approaches. This review evaluates the methods through which redox proteins can be attached to electrode surfaces in a "wired" configuration, that is, one that facilitates direct electron transfer. The feasibility of simple electroactive adsorption onto a range of electrode surfaces is illustrated, with a highlight on the recent advances that have been achieved in biotechnological device construction using carbon materials and metal oxides. The covalent crosslinking strategies commonly used for the modification and biofunctionalization of electrode surfaces are also evaluated. Recent innovations in harnessing chemical biology methods for electrically wiring redox biology to surfaces are emphasized.

Interaction of the H-Cluster of FeFe Hydrogenase with Halides

FeFe hydrogenases catalyze H₂ oxidation and production using an "H-cluster", where two Fe ions are bound by an aza-dithiolate (adt) ligand. Various hypotheses have been proposed (by us and others) to explain that the enzyme reversibly inactivates under oxidizing, anaerobic conditions: intramolecular binding of the N atom of adt, formation of the so-called "Hox/inact" state or nonproductive binding of H₂ to isomers of the H-cluster. Here, we show that none of the above explains the new finding that the anaerobic, oxidative, H₂-dependent reversible inactivation is strictly dependent on the presence of Cl^- or Br^-. We provide experimental evidence that chloride uncompetitively inhibits the enzyme: it reversibly binds to catalytic intermediates of H₂ oxidation (but not to the resting "Hox" state), after which oxidation locks the active site into a stable, saturated, inactive form, the structure of which is proposed here based on DFT calculations. The halides interact with the amine group of the H-cluster but do not directly bind to iron. It should be possible to stabilize the inhibited state in amounts compatible with spectroscopic investigations to explore further this unexpected reactivity of the H-cluster of hydrogenase.

Development of air-stable hydrogen evolution catalysts

Obtaining abundant pure hydrogen by reduction of water has an important implication in the development of clean and renewable energy. Hence research focused on the development of non-noble metal based facile and energy efficient catalysts for proton reduction is on the rise. However, for practical utilization, it is necessary that these complexes function unabated in the presence of atmospheric oxygen and other common contaminants in abundant water sources. There has been very little activity towards the development of oxygen-tolerant hydrogen producing catalysts. This article aims to draw attention to this issue of oxygen sensitivity in the HER and highlights the development of a few air-stable HER catalysts (enzymatic as well as artificial) elaborating the challenges involved and the techniques discovered to overcome this significant deterrent to large-scale hydrogen production by electrolysis from abundant water sources.

Protein bioelectronics: a review of what we do and do not know

We review the status of protein-based molecular electronics. First, we define and discuss fundamental concepts of electron transfer and transport in and across proteins and proposed mechanisms for these processes. We then describe the immobilization of proteins to solid-state surfaces in both nanoscale and macroscopic approaches, and highlight how different methodologies can alter protein electronic properties. Because immobilizing proteins while retaining biological activity is crucial to the successful development of bioelectronic devices, we discuss this process at length. We briefly discuss computational predictions and their connection to experimental results. We then summarize how the biological activity of immobilized proteins is beneficial for bioelectronic devices, and how conductance measurements can shed light on protein properties. Finally, we consider how the research to date could influence the development of future bioelectronic devices.

Spectroscopic Evidence of Reversible Disassembly of the [FeFe] Hydrogenase Active Site

[FeFe] hydrogenases are extremely active and efficient H₂-converting biocatalysts. Their active site comprises a unique [2Fe] subcluster bonded to a canonical [4Fe-4S] cluster. The [2Fe] subsite can be introduced into hydrogenases lacking an assembled H-cluster through incubation with a synthesized [2Fe]_H precursor, which initially produces the CO-inhibited state of the enzyme. We present FTIR spectroelectrochemical studies on the CO-inhibited state of the [FeFe] hydrogenase from Desulfovibrio desulfuricans, DdHydAB. At very negative potentials, disassembly of the H-cluster and dissociation of the [2Fe] subcluster is observed. Subsequently raising the potential allows cofactor rebinding and H-cluster reassembly. This demonstrates how the stability of the [2Fe]-[4Fe-4S] intercluster bond depends on the applied potential and the presence of an inhibiting CO ligand on the [2Fe] subcluster. These results provide insight into the mechanisms of CO inhibition and H-cluster assembly in [FeFe] hydrogenases. A fundamental understanding of these properties will provide clues for designing better H₂-converting catalysts.

Electrochemical Investigations of the Mechanism of Assembly of the Active-Site H-Cluster of [FeFe]-Hydrogenases

Protein film electrochemistry (PFE) has been used to study the assembly of the complex 6Fe active site of [FeFe]-hydrogenases (known as the H-cluster) from its precursors-the [4Fe-4S] domain that is already coordinated within the host, and the 2Fe domain that is presented as a synthetic water-soluble complex stabilized by an additional CO. Not only does PFE allow control of redox states via the electrode potential but also the immobilized state of the enzyme facilitates control of extremely low concentrations of the 2Fe complex. Results for two enzymes, CrHydA1 from Chlamydomonas reinhardtii and CpI from Clostridium pasteurianum, are very similar, despite large differences in size and structure. Assembly begins with very tight binding of the 34-valence electron 2Fe complex to the apo-[4Fe-4S] enzyme, well before the rate-determining step. The precursor is trapped under highly reducing conditions (<-0.5 V vs SHE) that prevent fusion of the [4Fe-4S] and 2Fe domains (via cysteine-S) since the immediate product would be too electron-rich. Relaxing this condition allows conversion to the active H-cluster. The intramolecular steps are relevant to the final stage of biological H-cluster maturation.

Following [FeFe] Hydrogenase Active Site Intermediates by Time-Resolved Mid-IR Spectroscopy

Time-resolved nanosecond mid-infrared spectroscopy is for the first time employed to study the [FeFe] hydrogenase from Chlamydomonas reinhardtii and to investigate relevant intermediates of the enzyme active site. An actinic 355 nm, 10 ns laser flash triggered photodissociation of a carbonyl group from the CO-inhibited state Hox-CO to form the state Hox, which is an intermediate of the catalytic proton reduction cycle. Time-resolved infrared spectroscopy allowed us to directly follow the subsequent rebinding of the carbonyl, re-forming Hox-CO, and determine the reaction half-life to be t1/2 ≈ 13 ± 5 ms at room temperature. This gives direct information on the dynamics of CO inhibition of the enzyme.

Proton-coupled electron transfer dynamics in the catalytic mechanism of a [NiFe]-hydrogenase

The movement of protons and electrons is common to the synthesis of all chemical fuels such as H2. Hydrogenases, which catalyze the reversible reduction of protons, necessitate transport and reactivity between protons and electrons, but a detailed mechanism has thus far been elusive. Here, we use a phototriggered chemical potential jump method to rapidly initiate the proton reduction activity of a [NiFe] hydrogenase. Coupling the photochemical initiation approach to nanosecond transient infrared and visible absorbance spectroscopy afforded direct observation of interfacial electron transfer and active site chemistry. Tuning of intramolecular proton transport by pH and isotopic substitution revealed distinct concerted and stepwise proton-coupled electron transfer mechanisms in catalysis. The observed heterogeneity in the two sequential proton-associated reduction processes suggests a highly engineered protein environment modulating catalysis and implicates three new reaction intermediates; Nia-I, Nia-D, and Nia-SR(-). The results establish an elementary mechanistic understanding of catalysis in a [NiFe] hydrogenase with implications in enzymatic proton-coupled electron transfer and biomimetic catalyst design.

[FeFe]- and [NiFe]-hydrogenase diversity, mechanism, and maturation

The [FeFe]- and [NiFe]-hydrogenases catalyze the formal interconversion between hydrogen and protons and electrons, possess characteristic non-protein ligands at their catalytic sites and thus share common mechanistic features. Despite the similarities between these two types of hydrogenases, they clearly have distinct evolutionary origins and likely emerged from different selective pressures. [FeFe]-hydrogenases are widely distributed in fermentative anaerobic microorganisms and likely evolved under selective pressure to couple hydrogen production to the recycling of electron carriers that accumulate during anaerobic metabolism. In contrast, many [NiFe]-hydrogenases catalyze hydrogen oxidation as part of energy metabolism and were likely key enzymes in early life and arguably represent the predecessors of modern respiratory metabolism. Although the reversible combination of protons and electrons to generate hydrogen gas is the simplest of chemical reactions, the [FeFe]- and [NiFe]-hydrogenases have distinct mechanisms and differ in the fundamental chemistry associated with proton transfer and control of electron flow that also help to define catalytic bias. A unifying feature of these enzymes is that hydrogen activation itself has been restricted to one solution involving diatomic ligands (carbon monoxide and cyanide) bound to an Fe ion. On the other hand, and quite remarkably, the biosynthetic mechanisms to produce these ligands are exclusive to each type of enzyme. Furthermore, these mechanisms represent two independent solutions to the formation of complex bioinorganic active sites for catalyzing the simplest of chemical reactions, reversible hydrogen oxidation. As such, the [FeFe]- and [NiFe]-hydrogenases are arguably the most profound case of convergent evolution. This article is part of a Special Issue entitled: Fe/S proteins: Analysis, structure, function, biogenesis and diseases.

New reactions of terminal hydrides on a diiron dithiolate

Mechanisms for biological and bioinspired dihydrogen activation and production often invoke the intermediacy of diiron dithiolato dihydrides. The first example of such a Fe2(SR)2H2 species is provided by the complex [(term-H)(μ-H)Fe2(pdt)(CO)(dppv)2] ([H1H](0)). Spectroscopic and computational studies indicate that [H1H](0) contains both a bridging hydride and a terminal hydride, which, notably, occupies a basal site. The synthesis begins with [(μ-H)Fe2(pdt)(CO)2(dppv)2](+) ([H1(CO)](+)), which undergoes substitution to afford [(μ-H)Fe2(pdt)(CO)(NCMe)(dppv)2](+) ([H1(NCMe)](+)). Upon treatment of [H1(NCMe)](+) with borohydride salts, the MeCN ligand is displaced to afford [H1H](0). DNMR (EXSY, SST) experiments on this complex show that the terminal and bridging hydride ligands interchange intramolecularly at a rate of 1 s(-1) at -40 °C. The compound reacts with D2 to afford [D1D](0), but not mixed isotopomers such as [H1D](0). The dihydride undergoes oxidation with Fc(+) under CO to give [1(CO)](+) and H2. Protonation in MeCN solution gives [H1(NCMe)](+) and H2. Carbonylation converts [H1H](0) into [1(CO)](0).

The oxidative inactivation of FeFe hydrogenase reveals the flexibility of the H-cluster

Nature is a valuable source of inspiration in the design of catalysts, and various approaches are used to elucidate the mechanism of hydrogenases, the enzymes that oxidize or produce H2. In FeFe hydrogenases, H2 oxidation occurs at the H-cluster, and catalysis involves H2 binding on the vacant coordination site of an iron centre. Here, we show that the reversible oxidative inactivation of this enzyme results from the binding of H2 to coordination positions that are normally blocked by intrinsic CO ligands. This flexibility of the coordination sphere around the reactive iron centre confers on the enzyme the ability to avoid harmful reactions under oxidizing conditions, including exposure to O2. The versatile chemistry of the diiron cluster in the natural system might inspire the design of novel synthetic catalysts for H2 oxidation.

Electron transfer kinetics in CdS nanorod-[FeFe]-hydrogenase complexes and implications for photochemical H₂ generation

This Article describes the electron transfer (ET) kinetics in complexes of CdS nanorods (CdS NRs) and [FeFe]-hydrogenase I from Clostridium acetobutylicum (CaI). In the presence of an electron donor, these complexes produce H2 photochemically with quantum yields of up to 20%. Kinetics of ET from CdS NRs to CaI play a critical role in the overall photochemical reactivity, as the quantum efficiency of ET defines the upper limit on the quantum yield of H2 generation. We investigated the competitiveness of ET with the electron relaxation pathways in CdS NRs by directly measuring the rate and quantum efficiency of ET from photoexcited CdS NRs to CaI using transient absorption spectroscopy. This technique is uniquely suited to decouple CdS→CaI ET from the processes occurring in the enzyme during H2 production. We found that the ET rate constant (k(ET)) and the electron relaxation rate constant in CdS NRs (k(CdS)) were comparable, with values of 10(7) s(-1), resulting in a quantum efficiency of ET of 42% for complexes with the average CaI:CdS NR molar ratio of 1:1. Given the direct competition between the two processes that occur with similar rates, we propose that gains in efficiencies of H2 production could be achieved by increasing k(ET) and/or decreasing k(CdS) through structural modifications of the nanocrystals. When catalytically inactive forms of CaI were used in CdS-CaI complexes, ET behavior was akin to that observed with active CaI, demonstrating that electron injection occurs at a distal iron-sulfur cluster and is followed by transport through a series of accessory iron-sulfur clusters to the active site of CaI. Using insights from this time-resolved spectroscopic study, we discuss the intricate kinetic pathways involved in photochemical H2 generation in CdS-CaI complexes, and we examine how the relationship between the electron injection rate and the other kinetic processes relates to the overall H2 production efficiency.

Electronic and molecular structures of the active-site H-cluster in [FeFe]-hydrogenase determined by site-selective X-ray spectroscopy and quantum chemical calculations

Site-selective X-ray spectroscopy discriminated the cubane and diiron units in the H-cluster of [FeFe]-hydrogenase revealing its electronic and structural configurations.

Direct electrochemistry of Shewanella oneidensis cytochrome c nitrite reductase: evidence of interactions across the dimeric interface

Shewanella oneidensis cytochrome c nitrite reductase (soNrfA), a dimeric enzyme that houses five c-type hemes per protomer, conducts the six-electron reduction of nitrite and the two-electron reduction of hydroxylamine. Protein film voltammetry (PFV) has been used to study the cytochrome c nitrite reductase from Escherichia coli (ecNrfA) previously, revealing catalytic reduction of both nitrite and hydroxylamine substrates by ecNrfA adsorbed to a graphite electrode that is characterized by "boosts" and attenuations in activity depending on the applied potential. Here, we use PFV to investigate the catalytic properties of soNrfA during both nitrite and hydroxylamine turnover and compare those properties to the properties of ecNrfA. Distinct differences in both the electrochemical and kinetic characteristics of soNrfA are observed; e.g., all detected electron transfer steps are one-electron in nature, contrary to what has been observed in ecNrfA [Angove, H. C., Cole, J. A., Richardson, D. J., and Butt, J. N. (2002) J. Biol. Chem. 277, 23374-23381]. Additionally, we find evidence of substrate inhibition during nitrite turnover and negative cooperativity during hydroxylamine turnover, neither of which has previously been observed in any cytochrome c nitrite reductase. Collectively, these data provide evidence that during catalysis, potential pathways of communication exist between the individual soNrfA monomers comprising the native homodimer.

Covalent attachment of FeFe hydrogenases to carbon electrodes for direct electron transfer

Direct electron transfer between enzymes and electrodes is now commonly achieved, but obtaining protein films that are very stable may be challenging. This is particularly crucial in the case of hydrogenases, the enzymes that catalyze the biological conversion between dihydrogen and protons, because the instability of the hydrogenase films may prevent the use of these enzymes as electrocatalysts of H(2) oxidation and production in biofuel cells and photoelectrochemical cells. Here we show that two different FeFe hydrogenases (from Chamydomonas reinhardtii and Clostridium acetobutylicum) can be covalently attached to functionalized pyrolytic graphite electrodes using peptidic coupling. In both cases, a surface patch of lysine residues makes it possible to favor an orientation that is efficient for fast, direct electron transfer. High hydrogen-oxidation current densities are maintained for up to one week, the only limitation being the intrinsic stability of the enzyme. We also show that covalent attachment has no effect on the catalytic properties of the enzyme, which means that this strategy can also used be for electrochemical studies of the catalytic mechanism.

Electrocatalytic mechanism of reversible hydrogen cycling by enzymes and distinctions between the major classes of hydrogenases

The extraordinary ability of Fe- and Ni-containing enzymes to catalyze rapid and efficient H(+)/H(2) interconversion--a property otherwise exclusive to platinum metals--has been investigated in a series of experiments combining variable-temperature protein film voltammetry with mathematical modeling. The results highlight important differences between the catalytic performance of [FeFe]-hydrogenases and [NiFe]-hydrogenases and justify a simple model for reversible catalytic electron flow in enzymes and electrocatalysts that should be widely applicable in fields as diverse as electrochemistry, catalysis, and bioenergetics. The active site of [FeFe]-hydrogenases, an intricate Fe-carbonyl complex known as the "H cluster," emerges as a supreme catalyst.

Combining acid-base, redox and substrate binding functionalities to give a complete model for the [FeFe]-hydrogenase

Some enzymes function by coupling substrate turnover with electron transfer from a redox cofactor such as ferredoxin. In the [FeFe]-hydrogenases, nature's fastest catalysts for the production and oxidation of H(2), the one-electron redox by a ferredoxin complements the one-electron redox by the diiron active site. In this Article, we replicate the function of the ferredoxins with the redox-active ligand Cp*Fe(C(5)Me(4)CH(2)PEt(2)) (FcP*). FcP* oxidizes at mild potentials, in contrast to most ferrocene-based ligands, which suggests that it might be a useful mimic of ferredoxin cofactors. The specific model is Fe(2)[(SCH(2))(2)NBn](CO)(3)(FcP*)(dppv) (1), which contains the three functional components of the active site: a reactive diiron centre, an amine as a proton relay and, for the first time, a one-electron redox module. By virtue of the synthetic redox cofactor, [1](2+) exhibits unique reactivity towards hydrogen and CO. In the presence of excess oxidant and base, H(2) oxidation by [1](2+) is catalytic.

The [NiFe]-hydrogenase of the cyanobacterium Synechocystis sp. PCC 6803 works bidirectionally with a bias to H2 production

Protein film electrochemistry (PFE) was utilized to characterize the catalytic activity and oxidative inactivation of a bidirectional [NiFe]-hydrogenase (HoxEFUYH) from the cyanobacterium Synechocystis sp. PCC 6803. PFE provides precise control of the redox potential of the adsorbed enzyme so that its activity can be monitored under changing experimental conditions as current. The properties of HoxEFUYH are different from those of both the standard uptake and the "oxygen-tolerant" [NiFe]-hydrogenases. First, HoxEFUYH is biased toward proton reduction as opposed to hydrogen oxidation. Second, despite being expressed under aerobic conditions in vivo, HoxEFUYH is clearly not oxygen-tolerant. Aerobic inactivation of catalytic hydrogen oxidation by HoxEFUYH is total and nearly instantaneous, producing two inactive states. However, unlike the Ni-A and Ni-B inactive states of standard [NiFe]-hydrogenases, both of these states are quickly (<90 s) reactivated by removal of oxygen and exposure to reducing conditions. Third, proton reduction continues at 25-50% of the maximal rate in the presence of 1% oxygen. Whereas most previously characterized [NiFe]-hydrogenases seem to be preferential hydrogen oxidizing catalysts, the cyanobacterial enzyme works effectively in both directions. This unusual catalytic bias as well as the ability to be quickly reactivated may be essential to fulfilling the physiological role in cyanobacteria, organisms expected to experience swings in cellular reduction potential as they switch between aerobic conditions in the light and dark anaerobic conditions. Our results suggest that the uptake [NiFe]-hydrogenases alone are not representative of the catalytic diversity of [NiFe]-hydrogenases, and the bidirectional heteromultimeric enzymes may serve as valuable models to understand the diverse mechanisms of tuning the reactivity of the hydrogen activating site.

Electrochemical kinetic investigations of the reactions of [FeFe]-hydrogenases with carbon monoxide and oxygen: comparing the importance of gas tunnels and active-site electronic/redox effects

A major obstacle for future biohydrogen production is the oxygen sensitivity of [FeFe]-hydrogenases, the highly active catalysts produced by bacteria and green algae. The reactions of three representative [FeFe]-hydrogenases with O(2) have been studied by protein film electrochemistry under conditions of both H(2) oxidation and H(2) production, using CO as a complementary probe. The hydrogenases are DdHydAB and CaHydA from the bacteria Desulfovibrio desulfuricans and Clostridium acetobutylicum , and CrHydA1 from the green alga Chlamydomonas reinhardtii . Rates of inactivation depend on the redox state of the active site 'H-cluster' and on transport through the protein to reach the pocket in which the H-cluster is housed. In all cases CO reacts much faster than O(2). In the model proposed, CaHydA shows the most sluggish gas transport and hence little dependence of inactivation rate on H-cluster state, whereas DdHydAB shows a large dependence on H-cluster state and the least effective barrier to gas transport. All three enzymes show a similar rate of reactivation from CO inhibition, which increases upon illumination: the rate-determining step is thus assigned to cleavage of the labile Fe-CO bond, a reaction likely to be intrinsic to the atomic and electronic state of the H-cluster and less sensitive to the surrounding protein.

Dynamic electrochemical experiments on hydrogenases

A powerful approach for studying hydrogenases, applying a suite of dynamic electrochemical techniques known as protein film electrochemistry, is trailblazing fresh discoveries and providing a wealth of quantitative data on these complex enzymes. The information now stemming from experiments on tiny quantities of hydrogenases ranges from their kinetics and catalytic bias (a preference to operate in H(2) oxidation vs. H(2) production) to wide differences in the ways they react with oxygen and other inhibitors. Tolerance of hydrogenase catalysis to oxygen is essential if organisms are to be exploited for photosynthetic hydrogen production, and is crucial in enabling aerobes to use trace H(2) as an energy source. Experiments described in this article may be adapted for other complex enzymes.

How oxygen attacks [FeFe] hydrogenases from photosynthetic organisms

Green algae such as Chlamydomonas reinhardtii synthesize an [FeFe] hydrogenase that is highly active in hydrogen evolution. However, the extreme sensitivity of [FeFe] hydrogenases to oxygen presents a major challenge for exploiting these organisms to achieve sustainable photosynthetic hydrogen production. In this study, the mechanism of oxygen inactivation of the [FeFe] hydrogenase CrHydA1 from C. reinhardtii has been investigated. X-ray absorption spectroscopy shows that reaction with oxygen results in destruction of the [4Fe-4S] domain of the active site H-cluster while leaving the di-iron domain (2Fe(H)) essentially intact. By protein film electrochemistry we were able to determine the order of events leading up to this destruction. Carbon monoxide, a competitive inhibitor of CrHydA1 which binds to an Fe atom of the 2Fe(H) domain and is otherwise not known to attack FeS clusters in proteins, reacts nearly two orders of magnitude faster than oxygen and protects the enzyme against oxygen damage. These results therefore show that destruction of the [4Fe-4S] cluster is initiated by binding and reduction of oxygen at the di-iron domain-a key step that is blocked by carbon monoxide. The relatively slow attack by oxygen compared to carbon monoxide suggests that a very high level of discrimination can be achieved by subtle factors such as electronic effects (specific orbital overlap requirements) and steric constraints at the active site.

Correcting for electrocatalyst desorption and inactivation in chronoamperometry experiments

Chronoamperometric experiments with adsorbed electrocatalysts are commonly performed either for analytical purposes or for studying the catalytic mechanism of a redox enzyme. In the context of amperometric sensors, the current may be recorded as a function of time while the analyte concentration is being increased to determine a linearity range. In mechanistic studies of redox enzymes, chronoamperometry proved powerful for untangling the effects of electrode potential and time, which are convoluted in cyclic voltammetric measurements, and for studying the energetics and kinetics of inhibition. In all such experiments, the fact that the catalyst's coverage and/or activity decreases over time distorts the data. This may hide meaningful features, introduce systematic errors, and limit the accuracy of the measurements. We propose a general and surprisingly simple method for correcting for electrocatalyst desorption and inactivation, which greatly increases the precision of chronoamperometric experiments. Rather than subtracting a baseline, this consists in dividing the current, either by a synthetic signal that is proportional to the instant electroactive coverage or by the signal recorded in a control experiment. In the latter, the change in current may result from film loss only or from film loss plus catalyst inactivation. We describe the different strategies for obtaining the control signal by analyzing various data recorded with adsorbed redox enzymes: nitrate reductase, NiFe hydrogenase, and FeFe hydrogenase. In each case we discuss the trustfulness and the benefit of the correction. This method also applies to experiments where electron transfer is mediated, rather than direct, providing the current is proportional to the time-dependent concentration of catalyst.